Hybrid vehicles and electric vehicles use an electric motor for driving the vehicle. The electric motor for such driving must have high output and efficiency as well as small size and weight. Therefore, it is required to reduce the weight and size on the order of grams and millimeters without lowering the output.
An electric motor has a coil that is formed by arranging a winding around a core of a stator. For achieving a higher output and a smaller size, it is required to improve a space factor of the coil in the stator or the like as far as possible. Also, a large current flows through the coil for increasing a magnetic force and thereby producing a large output.
The large current increases a quantity of heat that occurs due to a resistance of the winding. When a current value per sectional area of the winding is constant, the heat quantity per volume increases with increase in space factor described above. The heat generated in the winding moves across the neighboring winding portions, and is partially released from an outer layer of the coil. Also, another part of the heat is transferred to the core through an insulator, and is released from a release unit or the like arranged on an outer periphery of the stator. Further another part of the heat is transferred axially through the coil, and is released from a coil end.
The stator is configured to cool the outer layer of the coil and the outer periphery of the core by a cooling medium. However, when the heat generated inside the coil cannot be efficiently released to the outer periphery of the coil and the core, the internal temperature of the coil rises. Meanwhile, it is necessary to ensure electrical insulation between the core and the inner periphery of the coil as well as a function of protecting the winding. For this purpose, a cylindrical insulator made of resin is fitted to the core. Although this resin insulator has a high electrical insulation property, its thermal conductivity is low. Therefore, heat release to the core is suppressed so that the internal temperature of the coil tends to rise. The internal temperature rising of the winding adversely affects an insulation coating of the winding, and thereby lowers insulation properties, which may result in a problem that the output and the life of the electric motor lower.
For overcoming the above problems, it has been proposed to use a member of high thermal conductivity for forming the above insulator.    Patent Document 1: Japanese Patent Laying-Open No. 2001-128402